Optical measurements of properties in substances using propagation modes of light

ABSTRACT

This application describes designs, implementations, and techniques for controlling propagation mode or modes of light in a common optical path, which may include one or more waveguides, to sense a sample.

This application is a continuation application of co-pending U.S. patentapplication Ser. No. 10/860,094 entitled “Measurements of OpticalInhomogeneity and Other Properties in Substances Using Propagation Modesof Light” and filed Jun. 3, 2004, which was published as U.S. PatentApplication Publication No. 2005-0018202-A1 on Jan. 27, 2005.

The co-pending U.S. patent application Ser. No. 10/860,094 furtherclaims the benefits of the following four U.S. Provisional Applications:

1. Ser. No. 60/475,673 entitled “Method and Apparatus for AcquiringImages of Optical Inhomogeneity in Substances” and filed Jun. 4, 2003;

2. Ser. No. 60/514,768 entitled “Coherence-Gated Optical GlucoseMonitor” and filed Oct. 27, 2003;

3. Ser. No. 60/526,935 entitled “Method and Apparatus for AcquiringImages of Optical Inhomogeneity in Substances” and filed Dec. 4, 2003;and

4. Ser. No. 60/561,588 entitled “Acquiring Information of OpticalInhomogeneity and Other Properties in Substances” and filed Apr. 12,2004.

The entire disclosures of the above-referenced applications areincorporated herein by reference as part of this application.

BACKGROUND

This application relates to non-invasive, optical probing of varioussubstances, including but not limited to, skins, body tissues and organsof humans and animals.

Investigation of substances by non-invasive and optical means has beenthe object of many studies as inhomogeneity of light-matter interactionsin substances can reveal their structural, compositional, physiologicaland biological information. Various devices and techniques based onoptical coherence domain reflectometry (OCDR) may be used fornon-invasive optical probing of various substances, including but notlimited to skins, body tissues and organs of humans and animals, toprovide tomographic measurements of these substances.

In many OCDR systems, the light from a light source is split into asampling beam and a reference beam which propagate in two separateoptical paths, respectively. The light source may be partially coherentsource. The sampling beam is directed along its own optical path toimpinge on the substances under study, or sample, while the referencebeam is directed in a separate path towards a reference surface. Thebeams reflected from the sample and from the reference surface are thenbrought to overlap with each other to optically interfere. Because ofthe wavelength-dependent phase delay the interference results in noobservable interference fringes unless the two optical path lengths ofthe sampling and reference beams are very similar. This provides aphysical mechanism for ranging. A beam splitter may be used to split thelight from the light source and to combine the reflected sampling beamand the reflected reference beam for detection at an optical detector.This use of the same device for both splitting and recombining theradiation is essentially based on the well-known Michelsoninterferometer. The discoveries and the theories of the interference ofpartially coherent light are summarized by Born and Wolf in “Principlesof Optics”, Pergamon Press (1980).

Low-coherence light in free-space Michelson interferometers wereutilized for measurement purposes. Optical interferometers based onfiber-optic components were used in various instruments that uselow-coherence light as means of characterizing substances. Variousembodiments of the fiber-optic OCDR exist such as devices disclosed bySorin et al in U.S. Pat. No. 5,202,745, by Marcus et al in U.S. Pat. No.5,659,392, by Mandella et al in U.S. Pat. No. 6,252,666, and by Tearneyet al in U.S. Pat. No. 6,421,164. The application of OCDR in medicaldiagnoses in certain optical configurations has come to known as“optical coherence tomography” (OCT).

FIG. 1 illustrates a typical optical layout used in many fiber-opticOCDR systems described in the U.S. Pat. No. 6,421,164 and otherpublications. A fiber splitter is engaged to two optical fibers thatrespectively guide the sampling and reference beams in a Michelsonconfiguration. Common to many of these and other implementations, theoptical radiation from the low-coherence source is first physicallyseparated into two separate beams where the sampling beam travels in asample waveguide to interact with the sample while the reference beamtravels in a reference waveguide. The fiber splitter than combines thereflected radiation from the sample and the reference light from thereference waveguide to cause interference.

SUMMARY

The designs, techniques and exemplary implementations for non-invasiveoptical probing described in this application use the superposition andinterplay of different optical waves and modes propagating alongsubstantially the same optical path inside one or more common opticalwaveguides. When one of the optical waves or modes interacts with thesubstance under study its superposition with another wave or mode can beused for the purpose of acquiring information about the opticalproperties of the substance.

The methods and apparatus described in this application are at least inpart based on the recognition of various technical issues and practicalconsiderations in implementing OCDR in commercially practical and userfriendly apparatus, and various technical limitations in OCDR systemsdisclosed by the above referenced patents and other publications. As anexample, at least one of disadvantages associated to the OCDR systemdesigns shown in FIG. 1 or described in the aforementioned patents isthe separation of the reference light beam from the sample light beam.Due to the separation of the optical paths, the relative optical phaseor differential delay between the two beams may experience uncontrolledfluctuations and variations, such as different physical length,vibration, temperature, waveguide bending and so on. When the sample armis in the form of a fiber-based catheter that is separate from thereference arm, for example, the manipulation of the fiber may cause asignificant fluctuation and drift of the differential phase between thesample and reference light beams. This fluctuation and draft mayadversely affect the measurements. For example, the fluctuation anddrift in the differential phase between the two beams may lead totechnical difficulties in phase sensitive measurements as absolutevaluation of refractive indices and measurements of birefringence.

In various examples described in this application, optical radiation isnot physically separated to travel different optical paths. Instead, allpropagation waves and modes are guided along essentially the sameoptical path through one or more common optical waveguides. Such designswith the common optical path may be advantageously used to stabilize therelative phase among different radiation waves and modes in the presenceof environmental fluctuations in the system such as variations intemperatures, physical movements of the system especially of thewaveguides, and vibrations and acoustic impacts to the waveguides andsystem. In this and other aspects, the present systems are designed todo away with the two-beam-path configurations in variousinterferometer-based systems in which sample light and reference lighttravel in different optical paths in part to significantly reduce theabove fluctuation and drift in the differential phase delay. Therefore,the present systems have a “built-in” stability of the differentialoptical path by virtue of their optical designs and are beneficial forsome phase-sensitive measurement, such as the determination of theabsolute reflection phase and birefringence. In addition, the techniquesand devices described in this application simplify the structures andthe optical configurations of devices for optical probing by using thecommon optical path to guide light.

In various applications, it may be beneficial to acquire the absorptioncharacteristics of the material in an isolated volume inside the sample.In other case it may be desirable to map the distribution of somesubstances identifiable through their characteristic spectralabsorbance. In some OCDR systems such as systems in aforementionedpatents, it may be difficult to perform direct measurements of theoptical inhomogeneity with regard to these and other spectralcharacteristics. The systems and techniques described in thisapplication may be configured to allow for direct measurements of theseand other spectral characteristics of a sample.

Exemplary implementations are described below to illustrate variousfeatures and advantages of the systems and techniques. One of suchfeatures is methods and apparatus for acquiring information regardingoptical inhomogeneity in substance by a non-invasive means with the helpof a low-coherence radiation. Another feature is to achieve high signalstability and high signal-to-noise ratio by eliminating the need ofsplitting the light radiation into a sample path and a reference path.Additional features include, for example, a platform on whichphase-resolved measurements such as birefringence and absoluterefractive indices can be made, capability of acquiring opticalinhomogeneity with regard to the spectral absorbance, solving theproblem of signal drifting and fading caused by the polarizationvariation in various interferometer-based optical systems, and aneffective use of the source radiation with simple optical arrangements.Advantages of the systems and techniques described here include, amongothers, enhanced performance and apparatus reliability, simplifiedoperation and maintenance, simplified optical layout, reduced apparatuscomplexity, reduced manufacturing complexity and cost.

Various exemplary methods and techniques for optically sensing samplesare described. For example, one method for optically measuring a sampleincludes the following steps. A beam of guided light in a firstpropagation mode is directed to a sample. A first portion of the guidedlight in the first propagation mode is directed away from the sample ata location near the sample before the first portion reaches the sample.A second portion in the first propagation mode is directed to reach thesample. A reflection of the second portion from the sample is controlledto be in a second propagation mode different from the first propagationmode to produce a reflected second portion. Both the reflected firstportion in the first propagation mode and the reflected second portionin the second propagation mode are then directed through a commonwaveguide into a detection module to extract information from thereflected second portion on the sample.

Another method for optically measuring a sample is also described. Inthis method, light in a first propagation mode is directed to a vicinityof a sample under measurement. A first portion of the light in the firstpropagation mode is then directed to propagate away from the sample atthe vicinity of the sample without reaching the sample. A second portionof the light in the first propagation mode is directed to the sample tocause reflection at the sample. The reflected light from the sample iscontrolled to be in a second propagation mode that is independent fromthe first propagation mode to co-propagate with the first portion alonga common optical path. The first portion in the first propagation modeand the reflected light in the second propagation mode are used toobtain information of the sample.

This application further describes exemplary implementations of devicesand systems for optically measuring samples. One example of such devicesincludes a waveguide to receive and guide an input beam in a firstpropagation mode, and a probe head coupled to the waveguide to receivethe input beam and to reflect a first portion of the input beam back tothe waveguide in the first propagation mode and direct a second portionof the input beam to a sample. This probe head collects reflection ofthe second portion from the sample and exports to the waveguide thereflection as a reflected second portion in a second propagation modedifferent from the first propagation mode. This device further includesa detection module to receive the reflected first portion and thereflected second portion in the waveguide and to extract information ofthe sample carried by the reflected second portion.

In another example, an apparatus for optically measuring a sample isdisclosed to include a light source, a waveguide supporting at least afirst and a second independent propagation modes and guiding the lightradiation from the light source in the first propagation mode to thevicinity of a sample under examination, a probe head that terminates thewaveguide in the vicinity of the sample and reverses the propagationdirection of a portion of the first propagation mode in the waveguidewhile transmitting the remainder of the light radiation to the sample,the probe head operable to convert reflected light from the sample intothe second propagation mode, and a differential delay modulator thattransmits the light in both the first and the second propagation modesfrom the probe head and the waveguide and varies the relative opticalpath length between the first and the second propagation modes. In thisapparatus, a mode combiner is included to receive light from thedifferential delay modulator and operable to superpose the first and thesecond propagation modes by converting a portion of each mode to a pairof new modes. At least one photodetector is used in this apparatus toreceive light in at least one of the two new modes. Furthermore, anelectronic controller is used in communication with the photodetectorand is operable to extract information of the sample from the output ofthe photodetector.

In yet another example, a device is described to include an opticalwaveguide, an optical probe head and an optical detection module. Theoptical waveguide is to guide an optical radiation in a first opticalmode. The optical probe head is coupled to the optical waveguide toreceive the optical radiation. The optical probe head is operable to (1)redirect a portion of the optical radiation back to the opticalwaveguide while transmitting the remaining radiation to a sample, (2)receive and direct the reflected or backscattered radiation from thesample into the waveguide, and (3) control the reflected or thebackscattered light from the sample to be in a second optical modedifferent from the first optical mode. The optical detection module isused to receive the radiation redirected by the probe head through thewaveguide and to convert optical radiation in the first and secondoptical modes, at least in part, into a common optical mode.

A further example for a device for optically measuring a sample includesan input waveguide, an output waveguide and a probe head. The inputwaveguide supports a first and a second different propagation modes andis used to receive and guide an input beam in the first propagationmode. The output waveguide supports a first and a second differentpropagation modes. The probe head is coupled to the input waveguide toreceive the input beam and to the output waveguide to export light. Theprobe head is operable to direct a first portion of the input beam inthe first propagation mode into the output waveguide and direct a secondportion of the input beam to a sample. In addition, the probe headcollects reflection of the second portion from the sample and exports tothe output waveguide the reflection as a reflected second portion in thesecond propagation mode. Furthermore, this device includes a detectionmodule to receive the reflected first portion and the reflected secondportion in the output waveguide and to extract information of the samplecarried by the reflected second portion.

This application also describes an example of an apparatus for opticallymeasuring a sample. In this example, a first waveguide capable ofmaintaining at least one propagation mode is used. A light source thatemits radiation is used to excite the propagation mode in the firstwaveguide. A light director is used to terminate the first waveguidewith its first port, to pass the light mode entering the first port, atleast in part, through a second port, and to pass the light modesentering the second port, at least in part, through a third port. Theapparatus also includes a second waveguide that supports at least twoindependent propagation modes and having a first end coupled to thesecond port and a second end. Notably, a probe head is coupled to thesecond end of the second waveguide and operable to reverse thepropagation direction of the light in part back to the second waveguideand to transmit the remainder to the sample. This probe head is operableto transform the collected light from the sample reflection to anorthogonal mode supported by the second waveguide and direct light inthe orthogonal mode into the second waveguide. A third waveguide is alsoincluded which supports at least two independent propagation modes andis connected to the third port of the light director to receive lighttherefrom. A differential delay modulator is used to connect to thethird waveguide to receive light from the second waveguide and imposes avariable phase delay and a variable path length on one mode in referenceto the other. A fourth waveguide supporting at least two independentmodes is coupled to the differential delay modulator to receive lighttherefrom. A detection subsystem is positioned to receive light from thefourth waveguide and to superpose the two propagation modes from thefourth waveguide to form two new modes, mutually orthogonal. Thisdetection subsystem includes two photo-detectors respectively receivinglight in the new modes.

These and other features, system configurations, associated advantages,and implementation variations are described in detail in the attacheddrawings, the textual description, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an example of a conventional optical sensing device basedon the well-known Michelson interferometer with reference and samplebeams in two separate optical paths.

FIG. 2 shows one example of a sensing device according to oneimplementation.

FIG. 3 shows an exemplary implementation of the system depicted in FIG.2.

FIG. 4 shows one exemplary implementation of the probe head and oneexemplary implementation of the polarization-selective reflector (PSR)used in FIG. 3.

FIGS. 5A and 5B illustrate another exemplary optical sensing system thatuse three waveguides and a light director to direct light in two modesto and from the probe head in measuring a sample.

FIG. 6 illustrates the waveform of the intensity received at thedetector in the system in FIGS. 5A and 5B as a function of the phasewhere the detected light intensity exhibits an oscillating waveform thatpossesses a base frequency and its harmonics.

FIG. 7 shows one exemplary operation of the described system in FIG. 5Bor the system in FIG. 3 for acquiring images of optical inhomogeneity.

FIGS. 8A and 8B illustrate one exemplary design of the optical layout ofthe optical sensing system and its system implementation with anelectronic controller where light in a single mode is used as the inputlight.

FIG. 9 shows another example of a system implementation where theoptical probe head receives light in a single input mode and convertspart of light into a different mode.

FIGS. 10A and 10B show two examples of the possible designs for theprobe head used in sensing systems where the input light is in a singlemode.

FIG. 11 shows one implementation of a light director that includes apolarization-maintaining optical circulator and two polarization beamsplitters.

FIG. 12 illustrates an example of the optical differential delaymodulator used in present optical sensing systems where an externalcontrol signal is applied to control a differential delay element tochange and modulate the relative delay in the output.

FIGS. 12A and 12B illustrate two exemplary devices for implementing theoptical differential delay modulator in FIG. 12.

FIGS. 13A and 13B illustrate two examples of a mechanical variable delayelement suitable for implementing the optical differential delaymodulator shown in FIG. 12B.

FIG. 14A shows an exemplary implementation of the delay device in FIG.12B as part of or the entire differential delay modulator.

FIG. 14B shows a delay device based on the design in FIG. 14A where themirror and the variable optical delay line are implemented by themechanical delay device in FIG. 13A.

FIG. 15 illustrates an optical sensing system as an alternative to thedevice shown in FIG. 5B.

FIG. 16 shows a system based on the design in FIG. 2 where a tunablefilter is inserted in the input waveguide to filter the input light intwo different modes.

FIG. 17 shows another exemplary system based on the design in FIG. 8Awhere a tunable filter is inserted in the input waveguide to filter theinput light in a single mode.

FIG. 18 illustrates the operation of the tunable bandpass filter in thedevices in FIGS. 16 and 17.

FIG. 19A illustrates an example of a human skin tissue where the opticalsensing technique described here can be used to measure the glucoseconcentration in the dermis layer between the epidermis and thesubcutaneous layers.

FIG. 19B shows some predominant glucose absorption peaks in blood in awavelength range between 1 and 2.5 microns.

FIG. 20 illustrates one exemplary implementation of the detectionsubsystem in FIG. 3 where two diffraction gratings are used to separatedifferent spectral components in the output light beams from thepolarizing beam splitter.

DETAILED DESCRIPTION

Energy in light traveling in an optical path such as an opticalwaveguide may be in different propagation modes. Different propagationmodes may be in various forms. States of optical polarization of lightare examples of such propagation modes. Two independent propagationmodes do not mix with one another in the absence of a couplingmechanism. As an example, two orthogonally polarization modes do notinteract with each other even though the two modes propagate along thesame optical path or waveguide and are spatially overlap with eachother. The exemplary techniques and devices described in thisapplication use two independent propagation modes in light in the sameoptical path or waveguide to measure optical properties of a sample. Aprobe head may be used to direct the light to the sample, either in twopropagation modes or in a single propagation modes, and receive thereflected or back-scattered light from the sample.

For example, one beam of guided light in a first propagation mode may bedirected to a sample. A first portion of the first propagation mode maybe arranged to be reflected before reaching the sample while the asecond portion in the first propagation mode is allowed to reach thesample. The reflection of the second portion from the sample iscontrolled in a second propagation mode different from the firstpropagation mode to produce a reflected second portion. Both thereflected first portion in the first propagation mode and the reflectedsecond portion in the second propagation mode are directed through acommon waveguide into a detection module to extract information from thereflected second portion on the sample.

In another example, optical radiation in both a first propagation modeand a second, different propagation mode may be guided through anoptical waveguide towards a sample. The radiation in the firstpropagation mode is directed away from the sample without reaching thesample. The radiation in the second propagation mode is directed tointeract with the sample to produce returned radiation from theinteraction. Both the returned radiation in the second propagation modeand the radiation in the first propagation mode are coupled into theoptical waveguide away from the sample. The returned radiation in thesecond propagation mode and the radiation in the first propagation modefrom the optical waveguide are then used to extract information of thesample.

In these and other implementations based on the disclosure of thisapplication, two independent modes are confined to travel in the samewaveguides or the same optical path in free space except for the extradistance traveled by the probing light between the probe head and thesample. This feature stabilizes the relative phase, or differentialoptical path, between the two modes of light, even in the presence ofmechanical movement of the waveguides. This is in contrast tointerferometer sensing devices in which sample light and reference lighttravel in different optical paths. These interferometer sensing deviceswith separate optical paths are prone to noise caused by the variationin the differential optical path, generally complex in opticalconfigurations, and difficult to operate and implement. The examplesdescribed below based on waveguides are in part designed to overcomethese and other limitations.

FIG. 2 shows one example of a sensing device according to oneimplementation. This device directs light in two propagation modes alongthe same waveguide to an optical probe head near a sample 205 foracquiring information of optical inhomogeneity in the sample. A sampleholder may be used to support the sample 205 in some applications. Lightradiation from a broadband light source 201 is coupled into the firstdual-mode waveguide 271 to excite two orthogonal propagation modes, 001and 002. A light director 210 is used to direct the two modes to thesecond dual-mode waveguide 272 that is terminated by a probe head 220.The probe head 220 may be configured to perform at least the followingfunctions. The first function of the probe head 220 is to reverse thepropagation direction of a portion of light in the waveguide 272 in themode 001; the second function of the probe head 220 is to reshape anddeliver the remaining portion of the light in mode 002 to the sample205; and the third function of the probe head 220 is to collect thelight reflected from the sample 205 back to the second dual-modewaveguide 272. The back traveling light in both modes 001 and 002 isthen directed by light director 210 to the third waveguide 273 andfurther propagates towards differential delay modulator 250. Thedifferential delay modulator 250 is capable of varying the relativeoptical path length and optical phase between the two modes 001 and 002.A detection subsystem 260 is used to superpose the two propagation modes001 and 002 to form two new modes, mutually orthogonal, to be receivedby photo-detectors. Each new mode is a mixture of the modes 001 and 002.

The superposition of the two modes 001 and 002 in the detectionsubsystem 260 allows for a range detection. The light entering thedetection subsystem 260 in the mode 002 is reflected by the sample,bearing information about the optical inhomogeneity of the sample 205,while the other mode, 001, bypassing the sample 205 inside probe head220. So long as these two modes 001 and 002 remain independent throughthe waveguides their superposition in the detection subsystem 260 may beused to obtain information about the sample 205 without the separateoptical paths used in some conventional Michelson interferometersystems.

For the simplicity of the analysis, consider a thin slice of the sourcespectrum by assuming that the amplitude of the mode 001 is E₀₀₁ in afirst linear polarization and that of the mode 002 is E₀₀₂ in a second,orthogonal linear polarization in the first waveguide 271. The sample205 can be characterized by an effective reflection coefficient r thatis complex in nature; the differential delay modulator 350 can becharacterized by a pure phase shift Γ exerted on the mode 001. Let usnow superpose the two modes 001 and 002 by projecting them onto a pairof new modes, E_(A) and E_(B), by a relative 45-degree rotation in thevector space. The new modes, E_(A) and E_(B), may be expressed asfollowing: $\begin{matrix}\{ \begin{matrix}{{E_{A} = {\frac{1}{\sqrt{2}}( {{{\mathbb{e}}^{j\quad T}E_{001}} + {rE}_{002}} )}};} \\{E_{B} = {\frac{1}{\sqrt{2}}{( {{{\mathbb{e}}^{j\quad T}E_{001}} - {rE}_{002}} ).}}}\end{matrix}  & (1)\end{matrix}$It is assumed that all components in the system, except for the sample205, are lossless. The resultant intensities of the two superposed modesare $\begin{matrix}\{ \begin{matrix}{{I_{A} = {\frac{1}{2}\lbrack {E_{001}^{2} + E_{002}^{2} + {{r}E_{001}E_{002}\cos\quad( {\Gamma - \varphi} )}} \rbrack}};} \\{{I_{B} = {\frac{1}{2}\lbrack {E_{001}^{2} + E_{002}^{2} - {{r}E_{001}E_{002}\cos\quad( {\Gamma - \varphi} )}} \rbrack}},}\end{matrix}  & (2)\end{matrix}$where ψ is the phase delay associated with the reflection from thesample. A convenient way to characterize the reflection coefficient r isto measure the difference of the above two intensities, i.e.I _(A) −I _(B) =|r|E ₀₀₁ E ₀₀₂cos(Γ−ψ).   (3)If Γ is modulated by the differential delay modulator 250, the measuredsignal, Eq. (3), is modulated accordingly. For either a periodic or atime-linear variation of Γ, the measured responds with a periodicoscillation and its peak-to-peak value is proportional to the absolutevalue of r.

For a broadband light source 201 in FIG. 2, consider the two phases, Γand ψ to be dependent on wavelength. If the two modes 001 and 002experience significantly different path lengths when they reach thedetection system 260, the overall phase angle, Γ−ψ, should besignificantly wavelength dependant as well. Consequently the measuredsignal, being an integration of Eq. (3) over the source spectrum, yieldsa smooth function even though Γ is being varied. The condition for asignificant oscillation to occur in the measured signal is when the twomodes 001 and 002 experience similar path lengths at the location oftheir superposition. In this case the overall phase angle, Γ−ψ, becomeswavelength independent or nearly wavelength independent. In other words,for a given relative path length set by the modulator 250, anoscillation in the measured signal indicates a reflection, in the othermode, from a distance that equalizes the optical path lengths traveledby the two modes 001 and 002. Therefore the system depicted in FIG. 2can be utilized for ranging reflection sources.

Due to the stability of the relative phase between the two modes, 001and 002, phase-sensitive measurements can be performed with the systemin FIG. 2 with relative ease. The following describes an exemplarymethod based on the system in FIG. 2 for the determination of theabsolute phase associated with the radiation reflected from the sample205.

In this method, a sinusoidal modulation is applied to the differentialphase by the differential delay modulator 250, with a modulationmagnitude of M and a modulation frequency of Ω. The difference inintensity of the two new modes is the measured and can be expressed asfollows:I _(A) −I _(B) =|r|E ₀₀₁ E ₀₀₂cos[M sin(Ωt)−ψ].   (4)It is clear from Eq. (4) that the measured exhibits an oscillation at abase frequency of Ω and oscillations at harmonic frequencies of the basefrequency Ω. The amplitudes of the base frequency and each of theharmonics are related to Ω and |r|. The relationships between r and theharmonics can be derived. For instance, the amplitude of thebase-frequency oscillation and the second harmonic can be found from Eq.(4) to be:A _(Ω) =E ₀₀₁ E ₀₀₂ J ₁(M)|r|sinψ;   (5a)A _(2Ω) =E ₀₀₁ E ₀₀₂ J ₂(M)|r|cosψ,   (5b)where J₁ and J₂ are Bessel functions of the first and second order,respectively. Eq. (5a) and (5b) can be used to solve for |r| and ψ, i.e.the complete characterization of r. We can therefore completelycharacterize the complex reflection coefficient r by analyzing theharmonic content of various orders in the measured signal. Inparticular, the presence of the base-frequency component in the measuredis due to the presence of ψ.

FIG. 3 shows an exemplary implementation of the system depicted in FIG.2. The spectrum of source 201 may be chosen to satisfy the desiredranging resolution. The broader the spectrum is the better the rangingresolution. Various light sources may be used as the source 201. Forexample, some semiconductor superluminescent light emitting diodes(SLED) and amplified spontaneous emission (ASE) sources may possess theappropriate spectral properties for the purpose. In this particularexample, a polarization controller 302 may be used to control the stateof polarization in order to proportion the magnitudes of the two modes,001 and 002, in the input waveguide 371. The waveguide 371 and otherwaveguides 372 and 373 may be dual-mode waveguides and are capable ofsupporting two independent polarization modes which are mutuallyorthogonal. One kind of practical and commercially available waveguideis the polarization maintaining (PM) optical fiber. A polarizationmaintaining fiber can carry two independent polarization modes, namely,the s-wave polarized along its slow axis and the p-wave polarized alongits fast axis. In good quality polarization maintaining fibers these twomodes can have virtually no energy exchange, or coupling, forsubstantial distances. Polarization preserving circulator 310 directsthe flow of optical waves according to the following scheme: the twoincoming polarization modes from fiber 371 are directed into the fiber372; the two incoming polarization modes from fiber 372 are directed tothe fiber 373. A polarization-preserving circulator 310 may be used tomaintain the separation of the two independent polarization modes. Forinstance, the s-wave in the fiber 371 should be directed to the fiber372 as s-wave or p-wave only. Certain commercially availablepolarization-preserving circulators are adequate for the purpose.

The system in FIG. 3 implements an optical probe head 320 coupled to thewaveguide 372 for optically probing the sample 205. The probe head 320delivers a portion of light received from the waveguide 372, the lightin one mode (e.g., 002) of the two modes 001 and 002, to the sample 205and collects reflected and back-scattered light in the same mode 002from the sample 205. The returned light in the mode 002 collected fromthe sample 205 carries information of the sample 205 and is processed toextract the information of the sample 205. The light in the other mode001 in the waveguide 372 propagating towards the probe head 320 isreflected back by the probe head 320. Both the returned light in themode 002 and the reflected light in the mode 001 are directed back bythe probe head 320 into the waveguide 372 and to the differential delaymodulator 250 and the detection system 260 through the circulator 310and the waveguide 373.

In the illustrated implementation, the probe head 320 includes a lenssystem 321 and a polarization-selective reflector (PSR) 322. The lenssystem 321 is to concentrate the light energy into a small area,facilitating spatially resolved studies of the sample in a lateraldirection. The polarization-selective reflector 322 reflects the mode001 back and transmits the mode 002. Hence, the light in the mode 002transmits through the probe head 320 to impinge on the sample 205. Backreflected or scattered the light from the sample 205 is collected by thelens system 321 to propagate towards the circulator 310 along with thelight in the mode 001 reflected by PSR 322 in the waveguide 372.

FIG. 4 shows details of the probe head 320 and an example of thepolarization-selective reflector (PSR) 322 according to oneimplementation. The PSR 322 includes a polarizing beam splitter (PBS)423 and a reflector or mirror 424 in a configuration as illustratedwhere the PBS 423 transmits the selected mode (e.g., mode 002) to thesample 205 and reflects and diverts the other mode (e.g., mode 001) awayfrom the sample 205 and to the reflector 424. By retro reflection of thereflector 424, the reflected mode 001 is directed back to the PBS 423and the lens system 321. The reflector 424 may be a reflective coatingon one side of beam splitter 423. The reflector 424 should be aligned toallow the reflected radiation to re-enter the polarization-maintainingfiber 372. The transmitted light in the mode 002 impinges the sample 205and the light reflected and back scattered by the sample 205 in the mode002 transmits through the PBS 423 to the lens system 321. The lenssystem 321 couples the light in both the modes 001 and 002 into thefiber 372.

In the implementation illustrated in FIG. 3, the detection system 260includes a polarizing beam splitter 361, and two photodetectors 362 and363. The polarizing beam splitter 361 is used to receive the twoindependent polarization modes 001 and 002 from the modulator 250 andsuperposes the two independent polarization modes 001 and 002. The beamsplitter 361 may be oriented in such a way that, each independentpolarization is split into two parts and, for each independentpolarization mode, the two split portions possess the same amplitude.This way, a portion of the mode 001 and a portion of the mode 002 arecombined and mixed in each of the two output ports of the beam splitter361 to form a superposed new mode and each photodetector receives asuperposed mode characterized by Eq. (1). The polarizing beam splitter361 may be oriented so that the incident plane of its reflection surfacemakes a 45-degree angle with one of the two independent polarizationmode, 001 or 002.

The system in FIG. 3 further implements an electronic controller orcontrol electronics 370 to receive and process the detector outputs fromthe photodetectors 362 and 363 and to control operations of the systems.The electronic controller 370, for example, may be used to control theprobe head 320 and the differential delay modulator 250. Differentialdelay modulator 250, under the control of the electronics and programs,generates a form of differential phase modulation as the differentialpath length scans through a range that matches a range of depth insidethe sample 205. The electronic controller 370 may also be programmed torecord and extract the amplitude of the oscillation in the measuredsignal characterized by Eq. (3) at various differential path lengthsgenerated by the modulator 250. Accordingly, a profile of reflection asa function of the depth can be obtained as a one-dimensionalrepresentation of the sample inhomogeneity at a selected location on thesample 205.

For acquiring two-dimensional images of optical inhomogeneity in thesample 205, the probe head 320 may be controlled via a position scannersuch as a translation stage or a piezo-electric positioner so that theprobing light scans in a lateral direction, perpendicular to the lightpropagation direction. For every increment of the lateral scan a profileof reflection as a function of depth can be recorded with the methoddescribed above. The collected information can then be displayed on adisplay and interface module 372 to form a cross-sectional image thatreveals the inhomogeneity of the sample 205.

In general, a lateral scanning mechanism may be implemented in eachdevice described in this application to change the relative lateralposition of the optical probe head and the sample to obtain a2-dimensional map of the sample. A xy-scanner, for example, may beengaged either to the optical head or to a sample holder that holds thesample to effectuate this scanning in response to a position controlsignal generated from the electronic controller 370.

FIGS. 5A and 5B illustrate another exemplary system that use waveguides271, 272, and 273 and a light director 210 to direct light in two modesto and from the probe head 320 in measuring the sample 205. A firstoptical polarizer 510 is oriented with respect to the polarization axesof the PM waveguide 271 to couple radiation from the broadband lightsource 201 into the waveguide 271 in two orthogonal linear polarizationmodes as the independent propagation modes. An optical phase modulator520 is coupled in the waveguide 271 to modulate the optical phase oflight in one guided mode relative to the other. A variable differentialgroup delay (VDGD) device 530 is inserted in or connected to thewaveguide 273 to introduce a controllable amount of optical pathdifference between the two waves. A second optical polarizer 540 and anoptical detector 550 are used here to form a detection system. Thesecond polarizer 540 is oriented to project both of the guided wavesonto the same polarization direction so that the changes in optical pathdifference and the optical phase difference between the two propagationmodes cause intensity variations, detectable by the detector 550.

The light from the source 201 is typically partially polarized. Thepolarizer 510 may be aligned so that maximum amount of light from thesource 201 is transmitted and that the transmitted light is coupled toboth of the guided modes in the waveguide 271 with the substantiallyequal amplitudes. The electric fields for the two orthogonalpolarization modes S and P in the waveguide 271 can be expressed as:$\begin{matrix}\{ \begin{matrix}{{E_{s} = {\frac{1}{\sqrt{2}}E}},} \\{E_{p} = {\frac{1}{\sqrt{2}}{E.}}}\end{matrix}  & (6)\end{matrix}$where the electric field transmitting the polarizer is denoted as E. Itshould be appreciated that the light has a finite spectral width(broadband or partially coherent). The fields can be described by thefollowing Fourier integral:E=∫E _(ω) e ^(jωt) dω.   (7)For the simplicity of the analysis, a thin slice of the spectrum, i.e. alightwave of a specific wavelength, is considered below. Without loosinggenerality, it is assumed that all the components, including polarizers,waveguides, Router, PSR and VDGD, are lossless. Let us designate thereflection coefficient of the sample r, that is complex in nature. Thep-wave picks up an optical phase, Γ, relative to the s-wave as theyreach the second polarizer 540: $\begin{matrix}\{ \begin{matrix}{{E_{s} = {\frac{1}{\sqrt{2}}E}},} \\{E_{p} = {\frac{1}{\sqrt{2}}{rE}\quad{{\mathbb{e}}^{j\quad T}.}}}\end{matrix}  & (8)\end{matrix}$The light that passes through Polarizer 540 can be expressed by$\begin{matrix}{E_{a} = {{\frac{1}{\sqrt{2}}( {E_{s} + E_{p}} )} = {\frac{1}{2}E\quad{( {1 + {r\quad{\mathbb{e}}^{j\quad T}}} ).}}}} & (9)\end{matrix}$The intensity of the light that impinges on the photodetector 550 isgiven by: $\begin{matrix}{I = {{E_{a}E_{a}^{*}} = {\frac{1}{4}{{{E}^{2}\lbrack {1 + {r}^{2} + {2{r}\cos\quad( {\Gamma + \delta} )}} \rbrack}.}}}} & (10)\end{matrix}$where phase angle δ reflects the complex nature of the reflectioncoefficient of the sample 205 and is defined byr=|r|e ^(jδ).   (11)Assuming the modulator 520 exerts a sinusoidal phase modulation, withmagnitude M and frequency Ω, in the p-wave with respect to the s-wave,the light intensity received by the detector 550 can be expressed asfollows: $\begin{matrix}{I = {{\frac{1 + {r}^{2}}{4}{E}^{2}} + {\frac{r}{2}{E}^{2}{{\cos\quad\lbrack {{M\quad{\sin( {\Omega\quad t} )}} + \varphi + \delta} \rbrack}.}}}} & (12)\end{matrix}$where phase angle ψ is the accumulated phase slip between the two modes,not including the periodic modulation due to the modulator 520. The VDGD530 or a static phase shift in the modulator 520, may be used to adjustthe phase difference between the two modes to eliminate ψ.The waveform of I is graphically shown in FIG. 4.

FIG. 6 illustrates the waveform of the intensity I received at thedetector 550 as a function of the phase. The detected light intensityexhibits an oscillating waveform that possesses a base frequency of Ωand its harmonics. The amplitudes of the base frequency and each of theharmonics are related to δ and |r|. The mathematical expressions for therelationships between r and the harmonics can be derived. For instance,the amplitude of the base-frequency oscillation and the second harmonicare found to be:A _(Ω)=0.5|E| ² J ₁(M)|r|sin δ;   (13a)A _(2Ω)=0.5|E| ² J ₂(M)|r|cos δ,   (13b)where J₁ and J₂ are Bessel functions of the first and second order,respectively. Eq. (13a) and (13b) can be used to solve for |r| and 6,i.e. the complete characterization of r.

The effect of having a broadband light source 201 in the system in FIGS.5A and 5B is analyzed below. When there is a significant differentialgroup delay between the two propagation modes there must be anassociated large phase slippage ψ that is wavelength dependent. Asubstantial wavelength spread in the light source means that the phaseslippage also possesses a substantial spread. Such a phase spread cannotbe eliminated by a phase control device that does not also eliminate thedifferential group delay. In this case the detected light intensity isgiven by the following integral: $\begin{matrix}{I = {\int{\{ {{\frac{1 + {r}^{2}}{4}{{E(\lambda)}}^{2}} + {\frac{r}{2}{{E(\lambda)}}^{2}{\cos\quad\lbrack {{M\quad{\sin( {\Omega\quad t} )}} + {\varphi(\lambda)} + \delta} \rbrack}}} \}{{\mathbb{d}\lambda}.}}}} & (14)\end{matrix}$It is easy to see that if the range of ψ(λ) is comparable to π for thebandwidth of the light source no oscillation in I can be observed asoscillations for different wavelengths cancel out because of their phasedifference. This phenomenon is in close analogy to the interference ofwhite light wherein color fringes are visible only when the pathdifference is small (the film is thin). The above analysis demonstratesthat the use of a broadband light source enables range detection usingthe proposed apparatus. In order to do so, let the s-wave to have alonger optical path in the system compared to the p-wave (not includingits round-trip between Probing Head and Sample). For any given pathlength difference in the system there is a matching distance betweenProbing Head and Sample, z, that cancels out the path length difference.If an oscillation in I is observed the p-wave must be reflected fromthis specific distance z. By varying the path length difference in thesystem and record the oscillation waveforms we can therefore acquire thereflection coefficient r as a function of the longitudinal distance z,or depth. By moving Probing Head laterally, we can also record thevariation of r in the lateral directions.

FIG. 7 further shows one exemplary operation of the described system inFIG. 5B or the system in FIG. 3 for acquiring images of opticalinhomogeneity. At step 710, the relative phase delay between the twomodes is changed, e.g., increased by an increment, to a fixed value formeasuring the sample 205 at a corresponding depth. This may beaccomplished in FIG. 5B by using the differential delay device 530 orthe bias in the differential delay modulator 250 in FIG. 3. At step 720,a modulation driving signal is sent to the modulator 520 in FIG. 5B orthe modulator 250 in FIG. 3 to modulate the relative phase delay betweenthe two modes around the fixed value. At step 730, the intensitywaveform received in the detector 550 in FIG. 5B or the intensitywaveforms received in the detectors 362, 363 in FIG. 3 are measured andstored in the electronic controller 370. Upon completion of the step730, the electronic controller 370 controls the differential delaydevice 530 in FIG. 5B or the bias in the differential delay modulator250 in FIG. 3 to change the relative phase delay between the two modesto a different fixed value for measuring the sample 205 at a differentdepth. This process iterates as indicated by the processing loop 740until desired measurements of the sample at different depths at the samelocation are completed. At this point, electronic controller 370controls the probe head 320 to laterally move to a new location on thesample 205 and repeat the above measurements again until all desiredlocations on the sample 205 are completed. This operation is representedby the processing loop 750. The electronic controller 370 processes eachmeasurement to compute the values of δ and |r| from the base oscillationand the harmonics at step 760. Such data processing may be performedafter each measurement or after all measurements are completed. At step770, the computed data is sent to the display module 372.

In the above implementations, light for sensing the sample 205 is notseparated into two parts that travel along two different optical paths.Two independent propagation modes of the light are guided essentially inthe same waveguide at every location along the optical path except forthe extra distance traveled by one mode between the probe head 320 andthe sample 205. After redirected by the probe head 320, the two modesare continuously guided in the same waveguide at every location alongthe optical path to the detection module.

Alternatively, the light from the light source to the probe head may becontrolled in a single propagation mode (e.g., a first propagation mode)rather than two different modes. The probe head may be designed to causea first portion of the first mode to reverse its propagation directionwhile directing the remaining portion, or a second portion, to reach thesample. The reflection or back scattered light of the second portionfrom the sample is collected by the probe head and is controlled in thesecond propagation mode different from the first mode to produce areflected second portion. Both the reflected first portion in the firstpropagation mode and the reflected second portion in the secondpropagation mode are directed by the probe head through a commonwaveguide into the detection module for processing. In comparison withthe implementations that use light in two modes throughout the system,this alternative design further improves the stability of the relativephase delay between the two modes at the detection module and providesadditional implementation benefits.

FIGS. 8A and 8B illustrate one exemplary design of the optical layout ofthe optical sensing system and its system implementation with anelectronic controller. An input waveguide 871 is provided to directlight in a first propagation mode, e.g., the mode 001, from thebroadband light source 201 to a light director 810. The waveguide 871may be a mode maintaining waveguide designed to support at least onepropagation mode such as the mode 001 or 002. When light is coupled intothe waveguide 871 in a particular mode such as the mode 001, thewaveguide 871 essentially maintains the light in the mode 001. Apolarization maintaining fiber supporting two orthogonal linearpolarization modes, for example, may be used as the waveguide 871.Similar to systems shown in FIGS. 2, 3, 5A and 5B, dual-mode waveguides272 and 273 are used to direct the light. A light director 510 is usedto couple the waveguides 871, 272, and 273, to convey the mode 001 fromthe input waveguide 871 to one of the two modes (e.g., modes 001 and002) supported by the dual-mode waveguide 272, and to direct light intwo modes from the waveguide 272 to the dual-mode waveguide 273. In theexample illustrated in FIG. 8A, the light director 810 couples the lightin the mode 001 from the waveguide 871 into the same mode 001 in thewaveguide 272. Alternatively, the light director 810 may couple thelight in the mode 001 from the waveguide 871 into the different mode 002in the waveguide 272. The dual-mode waveguide 271 is terminated at theother end by a probe head 820 which couples a portion of light to thesample 205 for sensing.

The probe head 820 is designed differently from the prove head 320 inthat the probe head 830 converts part of light in the mode 001 into theother different mode 002 when the light is reflected or scattered backfrom the sample 205. Alternatively, if the light in the waveguide 272that is coupled from the waveguide 871 is in the mode 002, the probehead 820 converts that part of light in the mode 002 into the otherdifferent mode 001 when the light is reflected or scattered back fromthe sample 205. In the illustrated example, the probe head 820 performsthese functions: a) to reverse the propagation direction of a smallportion of the incoming radiation in mode 001; b) to reshape theremaining radiation and transmit it to the sample 205; and c) to convertthe radiation reflected from the sample 205 to an independent mode 002supported by the dual-mode waveguide 272. Since the probe head 820 onlyconverts part of the light into the other mode supported by thewaveguide 272, the probe head 820 is a partial mode converter in thisregard. Due to the operations of the probe head 820, there are two modespropagating away from the probe head 820, the mode 001 that bypasses thesample 205 and the mode 002 for light that originates from samplereflection or back scattering. From this point on, the structure andoperations of the rest of the system shown in FIG. 8A may be similar tothe systems in FIGS. 2, 3, 5A, and 5B.

FIG. 8B shows an exemplary implementation of the design in FIG. 8A wherean electronic controller 3370 is used to control the differential delaymodulator 250 and the probe head 820 and a display and interface module372 is provided. Radiation from broadband light source 201, which may bepartially polarized, is further polarized and controlled by an inputpolarization controller 802 so that only a single polarization mode isexcited in polarization-maintaining fiber 371 as the waveguide 871 inFIG. 8A. a polarization preserving circulator may be used to implementthe light director 810 for routing light from the waveguide 371 to thewaveguide 372 and from the waveguide 372 to the waveguide 373.

The probe head 820 in FIG. 8B may be designed to include a lens system821 similar to the lens system 321, a partial reflector 822, and apolarization rotator 823. The partial reflector 822 is used to reflectthe first portion of light received from the waveguide 372 back to thewaveguide 372 without changing its propagation mode and transmits lightto and from the sample 205. The polarization rotator 823 is used tocontrol the light from the sample 205 to be in the mode 002 upon entryof the waveguide 372.

FIG. 9 shows another example of a system implementation where theoptical probe head 820 receives light in a single input mode andconverts part of light into a different mode. An input polarizer 510 isused in the input PM fiber 272 to control the input light in the singlepolarization mode. A phase modulator 520 and a variable differentialgroup delay device 530 are coupled to the output PM fiver 273 to controland modulate the relative phase delay of the two modes before opticaldetection. An output polarizer 540 is provided to mix the two modes andthe detector 550 is used to detect the output from the output polarizer540.

FIGS. 10A and 10B show two examples of the possible designs for theprobe head 820 including a partially reflective surface 1010, a lenssystem 1020, and a quarter-wave plate 1030 for rotating the polarizationand to convert the mode. In FIG. 10A, the termination or end facet ofpolarization-maintaining fiber 372 is used as the partial reflector1010. An uncoated termination of an optical fiber reflects approximately4% of the light energy. Coatings can be used to alter the reflectivityof the termination to a desirable value. The lens system 1020 reshapesand delivers the remaining radiation to sample 205. The other roleplayed by the lens system 1020 is to collect the radiation reflectedfrom the sample 205 back into the polarization-maintaining fiber 372.The quarter wave plate 1030 is oriented so that its optical axis make a45-degree angle with the polarization direction of the transmittedlight. Reflected light from the sample 205 propagates through thequarter wave plate 1030 once again to become polarized in a directionperpendicular to mode 001, i.e. mode 002. Alternatively, the quarterwave plate 1030 may be replaced by a Faraday rotator. The head design inFIG. 10B changes the positions of the lens system 1020 and the quarterwave plate or Faraday rotator 1030.

In the examples in FIGS. 8A, 8B, and 9, there is only one polarizationmode entering the light director 810 or the polarization-preservingcirculator from waveguide 871 or 371. Therefore, the light director 810or the polarization preserving circulator may be constructed with apolarization-maintaining optical circulator 1110 and two polarizationbeam splitters 1120 and 1130 as shown in FIG. 11. Thepolarization-maintaining circulator 1110 is used to convey only onepolarization mode among its three ports, rather than both modes as inthe case shown in FIGS. 3, 5A and 5B. The polarizing beam splitter 1120and 1130 are coupled to polarization-maintaining circulator 1110 so thatboth polarization modes entering Port 2 are conveyed to Port 3 andremain independent.

A number of hardware choices are available for differential delaymodulator 250. FIG. 12 illustrates the general design of the modulator250 where an external control signal is applied to control adifferential delay element to change and modulate the relative delay inthe output. Either mechanical or non-mechanical elements may be used toproduce the desired relative delay between the two modes and themodulation on the delay.

In one implementation, a non-mechanical design may include one or moresegments of tunable birefringent materials such as liquid crystalmaterials or electro-optic birefringent materials such as lithiumniobate crystals in conjunction with one or more fixed birefringentmaterials such as quartz and rutile. The fixed birefringent materialprovides a fixed delay between two modes and the tunable birefringentmaterial provides the tuning and modulation functions in the relativedelay between the two modes. FIG. 12A illustrates an example of thisnon-mechanical design where the two modes are not physically separatedand are directed through the same optical path with birefringentsegments which alter the relative delay between two polarization modes.

FIG. 12B shows a different design where the two modes in the receivedlight are separated by a mode splitter into two different optical paths.A variable delay element is inserted in one optical path to adjust andmodulate the relative delay in response to an external control signal. Amode combiner is then used to combine the two modes together in theoutput. The mode splitter and the mode combiner may be polarizationbeams splitters when two orthogonal linear polarizations are used as thetwo modes.

The variable delay element in one of the two optical paths may beimplemented in various configurations. For example, the variable delayelement may be a mechanical element. A mechanical implementation of thedevice in FIG. 12B may be constructed by first separating the radiationby polarization modes with a polarizing beam splitter, one polarizationmode propagating through a fixed optical path while the otherpropagating through a variable optical path having a piezoelectricstretcher of polarization maintaining fibers, or a pair of collimatorsboth facing a mechanically movable retroreflector in such a way that thelight from one collimator is collected by the other through a trip toand from the retroreflector, or a pair collimators optically linkedthrough double passing a rotatable optical plate and bouncing off areflector.

FIGS. 13A and 13B illustrate two examples of a mechanical variable delayelement suitable for FIG. 12B. Such a mechanical variable delay devicemay be used to change the optical path length of a light beam at highspeeds and may have various applications other than what is illustratedin FIG. 12B. In addition, the optical systems in this application mayuse such a delay device.

The mechanical delay device shown in FIG. 13A includes an optical beamsplitter 1310, a rotating optical plate 1320 which may be a transparentplate, and a mirror or reflector 1330. The beam splitter 1310 is used asthe input port and the output port for the device. The rotating opticalplate 1320 is placed between the mirror 1330 and the beam splitter 1310.The input light beam 1300 is received by the beam splitter 1310 alongthe optical path directing from the beam splitter 1310 to the mirror1330 through the rotating optical plate 1320. A portion of the light1300 transmitting through the beam splitter 1310 is the beam 1301 whichimpinges on and transmits through the rotating optical plate 1320. Themirror or other optical reflector 1330 is oriented to be perpendicularto the light beam incident to the optical plate 1310 from the oppositeside. The reflected light beam 1302 from the mirror 1320 traces the sameoptical path back traveling until it encounters the Beam Splitter 1310.The Beam Splitter 1310 deflects part of the back traveling light 1302 toa different direction as the output beam 1303.

In this device, the variation of the optical path length is caused bythe rotation of the Optical Plate 1320. The Optical Plate 1320 may bemade of a good quality optical material. The two optical surfaces may beflat and well polished to minimize distortion to the light beam. Inaddition, the two surfaces should be parallel to each other so that thelight propagation directions on both sides of the Optical Plate 1320 areparallel. The thickness of the Optical Plate 1320 may be chosenaccording to the desirable delay variation and the range of the rotationangle. The optical path length experienced by the light beam isdetermined by the rotation angle of the Optical Plate 1320. When thesurfaces of the Optical Plate 1320 is perpendicular to the light beam(incident angle is zero), the path length is at its minimum. The pathlength increases as the incident angle increases.

In FIG. 13A, it may be beneficial to collimate the input light beam sothat it can travel the entire optical path without significantdivergence. The Optical Plate 1320 may be mounted on a motor forperiodic variation of the optical delay. A good quality mirror with aflat reflecting surface should be used to implement the mirror 1330. Thereflecting surface of the mirror 1330 may be maintained to beperpendicular to the light beam.

If a linearly polarized light is used as the input beam 1300 in FIG.13A, it is beneficial to have the polarization direction of the lightparallel to the incident plane (in the plane of the paper) as lessreflection occurs at the surfaces of Optical Plate 1320 for thispolarization compared to other polarization directions. Antireflectioncoatings can be used to further reduce the light reflection on thesurfaces of the Optical Plate 1320.

The beam splitter 1310 used in FIG. 13A uses both its opticaltransmission and optical reflection to direct light. This aspect of thebeam splitter 1310 causes reflection loss in the output of the devicedue to the reflection loss when the input light 1300 first enters thedevice through transmission of the beam splitter 1310 and thetransmission loss when the light exits the device through reflection ofthe beam splitter 1310. For example, a maximum of 25% of the total inputlight may be left in the output light if the beam splitter is a 50/50beam splitter. To avoid such optical loss, an optical circulator may beused in place of the beam splitter 1320. FIG. 13B illustrates an examplewhere the optical circulator 1340 with 3 ports is used to direct inputlight to the optical plate 1320 and the mirror 1330 and directs returnedlight to the output port. The optical circulator 1340 may be designed todirect nearly all light entering its port 1 to port 2 and nearly alllight entering its port 2 to the port 3 with nominal optical loss andhence significantly reduces the optical loss in the device. Commerciallyavailable optical circulators, either free-space or fiber-based, may beused to implement the circulator 1340.

FIG. 14A shows an exemplary implementation of the delay device in FIG.12B as part of or the entire differential delay modulator 250. A firstoptical mode splitter 1410 is used to separate two modes in thewaveguide 373 into two paths having two mirrors 1431 and 1432,respectively. A second optical mode splitter 1440, which is operated asa mode combiner, is used to combine the two modes into an output. If thetwo modes are two orthogonal linear polarizations, for example,polarization beam splitters may be used to implement the 1410 and 1440.A variable optical delay line or device 1420 is placed in the upper pathto control the differential delay between the two paths. The output maybe coupled into another dual-mode waveguide 1450 leading to thedetection module or directly sent into the detection module. FIG. 14Bshows a delay device based on the design in FIG. 14A where the mirror1432 and the variable optical delay line 1420 are implemented by themechanical delay device in FIG. 13A. The mechanical delay device in FIG.13B may also be used to implement the device in FIG. 14A.

In the above examples, a single dual-mode waveguide 272 or 372 is usedas an input and output waveguide for the probe head 220, 320, or 820.Hence, the input light, either in a single mode or two independentmodes, is directed into the probe head through that dual-mode waveguide272 or 372, and the output light in the two independent modes is alsodirected from the probe head to the detection subsystem or detector.

Alternatively, the single dual-mode waveguide 272 or 372 may be replacedby two separate waveguides, one to direct input light from the lightsource to the probe head and another to direct light from the probe headto the detection subsystem or detector. As an example, the device inFIG. 2 may have a second waveguide different from the waveguide 272 todirect reflected light in two different modes from the optical probehead 220 to the modulator 250 and the detection subsystem 260. In thisdesign, the light director 210 may be eliminated. This may be anadvantage. In implementation, the optics within the probe head may bedesigned to direct the reflected light in two modes to the secondwaveguide.

FIG. 15 illustrates an example for this design as an alternative to thedevice shown in FIG. 5B. In this design, the probing light is deliveredto the sample 205 through one dual-mode waveguide 1510 and thereflected/scattered light is collected by the probe head 320 and isdirected through another dual-mode waveguide 1520. With the probe headshown in FIG. 4, the mirror 424 may be oriented and aligned so that thelight is reflected into the waveguide 1520 instead of the waveguide1510. This design may be applied to other devices based on thedisclosure of this application, including the exemplary devices in FIGS.2, 3, 8A, 8B and 9.

The above-described devices and techniques may be used to obtain opticalmeasurements of a given location of the sample at different depths bycontrolling the relative phase delay between two modes at differentvalues and optical measurements of different locations of the sample toget a tomographic map of the sample at a given depth or various depthsby laterally changing the relative position of the probe head over thesample. Such devices and techniques may be further used to perform othermeasurements on a sample, including spectral selective measurements on alayer of a sample.

In various applications, it may be beneficial to obtain informationabout certain substances, identifiable through their spectralabsorbance, dispersed in the samples. For this purpose, a tunablebandpass filter may be used to either filter the light incident to theprobe head to select a desired spectral window within the broadbandspectrum of the incident light to measure the response of the sample andto vary the center wavelength of the spectral window to measure aspectral distribution of the responses of the sample. This tuning of thebandpass filter allows a variable portion of the source spectrum to passwhile measuring the distribution of the complex reflection coefficientof the sample.

Alternatively, the broadband light may be sent to the optical probe headwithout optical filtering and the spectral components at differentwavelengths in the output light from the probe head may be selected andmeasured to measure the response of the sample around a selectedwavelength or the spectral distribution of the responses of the sample.In one implementation, a tunable optical bandpass filter may be insertedin the optical path of the output light from the probe head to filterthe light. In another implementation, a grating or other diffractiveoptical element may be used to optically separate different spectralcomponents in the output light to be measured by the detection subsystemor the detector.

As an example, FIG. 16 shows a system based on the design in FIG. 2where a tunable filter 1610 is inserted in the input waveguide 271 tofilter the input light in two different modes. FIG. 17 shows anotherexemplary system based on the design in FIG. 8A where a tunable filter1710 is inserted in the input waveguide 871 to filter the input light ina single mode. Such a tunable filter may be placed in other locations.

FIG. 18 illustrates the operation of the tunable bandpass filter in thedevices in FIGS. 16 and 17. The filter selects a narrow spectral bandwithin the spectrum of the light source to measure the spectral featureof the sample.

Notably, the devices and techniques of this application may be used toselect a layer within a sample to measure by properly processing themeasured data. Referring back to the devices in FIGS. 16 and 17, let usassume that the absorption characteristics of a layer bounded byinterfaces I and II is to be measured. For the simplicity ofdescription, it is assumed that the spectral absorption of the substancein the layer is characterized by a wavelength-dependent attenuationcoefficient μ_(h) (λ) and that of other volume is characterized by μ_(g)(λ). It is further assumed that the substance in the vicinity ofinterface I (II) possesses an effective and wavelength independentreflection coefficient r_(I) (r_(II)). If the characteristic absorptionof interest is covered by the spectrum of the light source, an opticalfilter 1610 or 1710 with a bass band tunable across the characteristicabsorption of the sample 205 may be used to measure the spectralresponses of the sample 205 centered at different wavelengths.

In operation, the following steps may be performed. First, thedifferential delay modulator 250 is adjusted so that the path lengthtraveled by one mode (e.g., the mode 001) matches that of radiationreflected from interface I in the other mode (e.g., the mode 002). Atthis point, the pass band of filter 1610 or 1710 may be scanned whilerecording the oscillation of the measured signal due to a periodicdifferential phase generated by the modulator 250. The oscillationamplitude as a function of wavelength is given byA ₁(λ)=r ₁ e ^(−2μ) ^(x) _((λ)z) ^(I)   (15)where z_(I) is the distance of interface I measured from the top surfaceof the sample 205. Next, the differential delay modulator 250 isadjusted again to change the differential delay so that the path lengthtraveled by the mode 001 matches that of radiation reflected frominterface II in the mode 002. The measurement for the interface II isobtained as follows:A _(II)(λ)=r _(II) e ^(−2μ) ^(g) _((λ)z) ^(I) _(−2μ) ^(h) _((λ)z) ^(II),   (16)where z_(II) is the distance of interface II measured from interface I.To acquire the absorption characteristics of the layer bounded by theinterfaces I and II, Eq. (7) and Eq. (6) can be used to obtain thefollowing ratio: $\begin{matrix}{\frac{A_{II}(\lambda)}{A_{I}(\lambda)} = {\frac{r_{II}}{r_{I}}{{\mathbb{e}}^{{- 2}{\mu_{h}{(\lambda)}}z_{II}}.}}} & (17)\end{matrix}$Notably, this equation provides the information on the absorptioncharacteristics of the layer of interest only and this allowsmeasurement on the layer. This method thus provides a “coherence gating”mechanism to optically acquire the absorbance spectrum of a particularand designated layer beneath a sample surface.

It should be noted that the pass band of the optical filter 1610 or 1710may be designed to be sufficiently narrow to resolve the absorptioncharacteristics of interest and at the meantime broad enough todifferentiate the layer of interest. The following example formonitoring the glucose level by optically probing a patient's skin showsthat this arrangement is reasonable and practical.

Various dependable glucose monitors rely on taking blood samples fromdiabetes patients. Repeated pricking of skin can cause considerablediscomfort to patients. It is therefore desirable to monitor the glucoselevel in a noninvasive manner. It is well known that glucose in bloodpossesses “signature” optical absorption peaks in a near-infrared (NIR)wavelength range. It is also appreciated the main obstacle innoninvasive monitoring of glucose is due to the fact that a probinglight beam interacts, in its path, with various types of tissues andsubstances which possess overlapping absorption bands. Extracting thesignature glucose peaks amongst all other peaks has proven difficult.

The above “coherence gating” may be used overcome the difficulty inother methods for monitoring glucose. For glucose monitoring, thedesignated layer may be the dermis layer where glucose is concentratedin a network of blood vessels and interstitial fluid.

FIG. 19A illustrates an example of a human skin tissue where thecoherence gating technique described here can be used to measure theglucose concentration in the dermis layer between the epidermis and thesubcutaneous layers. The dermis layer may be optically selected andmeasured with the coherence gating technique. It is known that thesuperficial epidermis layer, owing to its pigment content, is thedominant source of NIR absorption. Because of the absence of blood,however, the epidermis yields no useful information for glucosemonitoring. The coherent gating technique can be applied to acquiresolely the absorbance spectrum of the dermis layer by rejecting theabsorptions of the epidermis and the subcutaneous tissues. An additionaladvantage of this technique is from the fact that dermis exhibits lesstemperature variation compared to the epidermis. It is known thatsurface temperature variation causes shifts of water absorption,hampering glucose monitoring.

FIG. 19B shows some predominant glucose absorption peaks in blood in awavelength range between 1 and 2.5 microns. The width of these peaks areapproximately 150 nm. To resolve the peaks, the bandwidth of the tunablebandpass filter may be chosen to be around 30 nm. The depth resolutionis determined by the following equation: $\begin{matrix}{{\frac{2{\ln(2)}}{\pi}\frac{\lambda_{o}^{2}}{\Delta\lambda}} = {60\mu\quad m}} & (18)\end{matrix}$Therefore, the coherence gating implemented with the devices in FIGS. 16and 17 or other optical sensing devices may be used to determine theabsorption characteristics of the glucose in tissue layers no less than60 μm thick. As illustrated in FIG. 19A, human skin consists of asuperficial epidermis layer that is typically 0.1 mm thick. Underneathepidermis is the dermis, approximately 1 mm thick, where glucoseconcentrates in blood and interstitial fluids. The above analysisindicates that it is possible to use the apparatus shown in FIGS. 16 and17 to isolate the absorption characteristics of the dermis from that ofthe epidermis and other layers.

It is clear from Eq. (18) that the product of spectral resolution andlayer resolution is a constant for a given center wavelength λ₀. Thechoice of the filter bandwidth should be made based on the tradeoffbetween these two resolutions against the specific requirements of themeasurement.

The tunable bandpass filter 1610 or 1710 may be operated to acquire theabsorption characteristics of an isolated volume inside a sample.

FIG. 20 illustrates one exemplary implementation of the detectionsubsystem 260 in FIG. 3 where two diffraction gratings 2010 and 2020 areused to separate different spectral components in the output light beamsfrom the polarizing beam splitter 361. A lens 2012 is positioned tocollect the diffracted components from the grating 2010 and focusdifferent spectral components to different locations on its focal plane.A detector array 2014 with multiple photodetector elements is placed atthe focal plane of the lens 2012 so that different spectral componentsare received by different photodetector elements. A second lens 2022 anda detector array 2024 are used in the optical path of the diffractedcomponents in a similar way. In devices shown in FIGS. 5A, 5B, 8A, and8B where a single optical detector is used for measurements, a singlegrating, a lens, and a detector array may be used.

In operation, each detector element receives light in a small wavelengthinterval. The photocurrents from all elements in an array can be summedto form a signal which is equivalent to the signal received in eachsingle detector without the grating shown in FIG. 3. By selectivelymeasuring the photocurrent from an individual element or a group ofelements in an array, the spectral information of the sample can beobtained.

Only a few implementations are disclosed in this application. However,it is understood that variations and enhancements may be made.

1. A method for optically measuring a sample, comprising: directing onebeam of guided light in a first propagation mode to a sample; directinga first portion of the guided light in the first propagation mode at alocation near the sample away from the sample before the first portionreaches the sample while allowing a second portion in the firstpropagation mode to reach the sample; controlling a reflection of thesecond portion from the sample to be in a second propagation modedifferent from the first propagation mode to produce a reflected secondportion; and directing both the reflected first portion in the firstpropagation mode and the reflected second portion in the secondpropagation mode through a common waveguide into a detection module toextract information from the reflected second portion on the sample. 2.The method as in claim 1, wherein the first propagation mode and thesecond propagation mode are two polarization modes that are orthogonalto each other.
 3. The method as in claim 2, wherein the firstpropagation mode and the second propagation mode are two orthogonallinear polarization modes.
 4. The method as in claim 2, furthercomprising using a polarization rotator to cause the reflected secondportion to be in the second propagation mode.
 5. The method as in claim2, further comprising using a Faraday rotator to cause the reflectedsecond portion to be in the second propagation mode.
 6. The method as inclaim 2, further comprising using a quarter wave plate to cause thereflected second portion to be in the second propagation mode.
 7. Themethod as in claim 1, further comprising using the common waveguide toboth direct the guided light to the sample and to guide the reflectedfirst portion and the reflected second portion away from the sample. 8.The method as in claim 7, wherein the waveguide is a polarizationmaintaining waveguide.
 9. The method as in claim 7, wherein thewaveguide is a polarization maintaining fiber.
 10. The method as inclaim 1, further comprising using an input waveguide different from thecommon waveguide to direct the guided light to the sample.
 11. Themethod as in claim 1, further comprising adjusting a relative lateralposition between the second portion and the sample to direct the secondportion to reach different locations on the sample to obtain informationat the different locations.
 12. A method for optically measuring asample, comprising: directing light in a first propagation mode to avicinity of a sample under measurement; directing a first portion of thelight in the first propagation mode to propagate away from the sample atthe vicinity of the sample without reaching the sample; directing asecond portion of the light in the first propagation mode to the sampleto cause reflection at the sample; controlling reflected light from thesample to be in a second propagation mode that is independent from thefirst propagation mode to co-propagate with the first portion along acommon optical path; and using the first portion in the firstpropagation mode and the reflected light in the second propagation modeto obtain information of the sample.
 13. The method as in claim 12,further comprising using at least one optical waveguide to guide light.14. The method as in claim 12, further comprising using at least oneoptical fiber to guide light.
 15. The method as in claim 12, wherein thefirst and the second propagation modes are polarization modes.
 16. Themethod as in claim 12, further comprising using a tunable opticalbandpass filter to filter the light before to the light reaches thevicinity of the sample to select a spectral response of the sample tomeasure.
 17. The method as in claim 12, further comprising using atunable optical bandpass filter to filter the first portion in the firstpropagation mode and the reflected light in the second propagation modeto select a spectral response of the sample to measure.
 18. A method,comprising: guiding optical radiation in both a first propagation modeand a second, different propagation mode through an optical waveguidetowards a sample; directing radiation in the first propagation mode awayfrom the sample without reaching the sample; directing radiation in thesecond propagation mode to interact with the sample to produce returnedradiation from the interaction; coupling both the returned radiation inthe second propagation mode and the radiation in the first propagationmode into the optical waveguide away from the sample; and using thereturned radiation in the second propagation mode and the radiation inthe first propagation mode from the optical waveguide to extractinformation of the sample.
 19. The method as in claim 18, wherein thefirst propagation mode and the second propagation mode are twopolarization modes that are orthogonal to each other.
 20. The method asin claim 18, further comprising adjusting a relative phase delay betweenthe radiation in the first propagation mode and the radiation in thesecond propagation mode that are directed in the optical waveguide awayfrom the sample to select a layer of the sample to measure.